Proximity switch (for example as described in SE520154 which is a type of inductive proximity switch) are used in different applications to detect the presence or absence of a second object in relation to a first object. For this purpose, the first object is provided with a proximity switch, while the second object is provided with a target device, that can be detected by the proximity switch. For example, proximity switches are often used in industrial and potentially dangerous environments for assisting in ensuring operational safety of a machine or an industrial process. It uses a combination of a switch unit and a target device to detect whether a gate, a door, a hatch or similar is closed in a safe mode, whereby a human operator is prevented from direct access to a potentially dangerous machine or process, or whether the gate, the door, the hatch or similar suddenly is opened.
The proximity switch of SE520154 is a contactless safety sensor (comprising a switch device and target) for gates, hatchers etc. A pulsating pulse train signal from a safety module or safety PLC (Programmable logic controller) is transmitted via the switch with a first carrier frequency to the target where the first carrier frequency is changed to a different carrier frequency that differs from the first one and sent back to the switch. The safeguard of the proximity switch described above is based on inversion of the received pulse train with an inverter which is located in the switch device.
The proximity switch as described in SE520154 has been accepted as fulfilling the highest safety category requirements, but has a serious drawback in that the inversion is located in the switch device and not in the target device. In case there is an error in the receiving unit in the switch device, the transmittal of the modulated pulse train from the transmitter unit in the switch device may be received by the receiving unit of the switch and wrongly interpreted as an indication that the target is present, although this is not the case. In other words, there is a risk of crosstalk between these modules (transmitter and receiver unit in the switch device). Consequently, the “own” send signal is demodulates “incorrectly” and the received pulse train is inverted in the switch device section and is then sent on to the controller without detection of errors.
An error of this kind, which might be a ground contact error in the receiver filter of the switch device, changes the properties of the receiver filter and allows frequencies that should have been blocked to pass.